Pulmonary wedge pressure, also sometimes referred to as pulmonary occlusion pressure, is an estimate of the left atrial, or left heart filling, pressure. The pulmonary wedge pressure has been widely used by clinicians as a valuable indication of a patient's cardiac output performance, and is especially useful during post-operative pharmaceutical and/or fluid titration in cardiovascular surgery patients. Under the current state of the art, wedge pressure is obtained by inserting a balloon flotation catheter, such as the well-known Swan-Ganz type flow-directed catheter, through the heart and into a smaller branch of the pulmonary artery. Once the catheter is in place, a balloon at the distal tip of the catheter is inflated to occlude blood flow through the branch. The pressure within the branch distal the occlusion, which is measured by a sensor in or near the tip of the catheter and distal the balloon, will then decay to some stable baseline value representative of the left heart filling pressure. The change over time of this filling pressure is then used by the clinician to judge the improvement, or lack thereof, in the patient's cardiac output.
Unfortunately, the conventional method for obtaining wedge pressure presents significant risk to the patient. Inflation of the catheter's balloon within the pulmonary artery causes a distinct force to be exerted outwardly against the artery's interior wall. A naturally fragile or otherwise weakened artery may not tolerate this outward force, resulting in rupture of the artery. Although not extraordinarily common, the ultimate effect of a ruptured artery is catastrophic to the patient; a surgical team has only between about 30 seconds and three minutes to open the patient's chest and clamp the bleeder before the patient bleeds to death into the plural cavity. To compound the problem, pulmonary artery catheters have a tendency to migrate downstream. As the catheter enters smaller and smaller portions of the arterial branch, the chance for rupture of the artery increases. Because of these inherent risks, the clinician's need for repeated wedge pressure measurements must be weighed against the jeopardy in which the patient is placed to obtain each and every measurement. The end result is a tendency to not often repeat the measurement except in the most acute cases, wherein the measurement is still only taken about every fifteen minutes. Because the greatest value for the pulmonary wedge pressure is derived from analysis of how the wedge pressure changes over time, the inability to obtain frequently repeated measurements presents a serious clinical limitation.
It is therefore a specific object of the present invention to obtain an estimate of the pulmonary wedge pressure without need for inflation of a pulmonary artery catheter balloon. It is a further object of the present invention to obtain such a measure, utilizing the beat-to-beat pulmonary artery blood pressure waveform, on a beat-by-beat basis in order to provide a more clinically useful indication of the dynamic aspects of a patient's cardiac condition. It is yet a further object of the present invention to provide an analysis machine for converting the pulmonary artery blood pressure waveform into the desired beat-by-beat estimate of pulmonary wedge, or occlusion, pressure.